Communication systems, and more specifically digital cellular radiotelephone systems, employ speech and channel coding techniques to represent analog speech in a digital manner. Essentially, a portion of a waveform of analog speech is sampled, digitized and compressed into bits which digitally represent the analog waveform. A block (or frame in time division multiple access, TDMA, systems) of the sampled bits are then sent to a channel coder where additional processing is performed. The resulting output of the channel coder undergoes further processing, depending on the particular digital radiotelephone system, and is then transmitted to a receiving station over an air interface.
Due to the effects of multi-path, Rayleigh fading, etc, the block of bits representing the original analog waveform of speech has a high probability of being corrupted when the communications link is operating at or near its designed signalnoise ratio. Typical digital cellular radiotelephone systems incorporate a cyclic redundancy check (CRC) whereby the channel coder within the transmitter performs a type of degenerate cyclic coding on a portion of the bits to be transmitted (the portion of bits which are coded are typically the most perceptually significant speech bits). This results in the addition of a predetermined number of CRC parity bits to form a CRC codeword. Receivers receiving the transmitted CRC codeword check the codeword, and if incorrect, determine that the entire block of bits is corrupted. Once this determination is made, the block (or frame) of bits is erased.
In the Global System for Mobile Communications (GSM) Pan-European Digital Cellular (PEDC) radiotelephone system, a 3-bit CRC codeword is implemented giving a (53,50) shortened cyclic block code. An important measure of link and receiver performance in GSM is the "Bad Frame Indicator" or logical BFI flag. The BFI flag is asserted when any of the information bits comprising the 50 bits to which the block code is applied are received in error. GSM Recommendation 5.05, titled "Radio Transmission and Reception," version 4.2.0 dated April, 1992, specifies that, on average, less than one undetected bad speech frame (i.e., the BFI flag is not asserted when an information bit is received in error) in ten seconds be measured under certain signaling conditions. It is well recognized that the 3-bit CRC codeword that has been originally implemented is insufficient to meet the BFI specifications as defined in GSM Recommendation 5.05 because the (53,50) code is not sufficiently powerful to detect many bit error patterns which form valid codewords, and hence satisfy the CRC syndrome calculation.
Additional criteria can, however, be derived from other forward error correction (FEC) statistics to form a composite BFI flag. One such criteria, in addition to the CRC codeword, is a bit correction threshold. This approach exploits the fact the bits protected by the CRC are also encoded using a convolutional code. It is possible to estimate the channel symbol error rate of the convolutionally coded symbols by re-encoding the information sequence recovered by convolutional decoding. The accumulated number of re-encoded channel symbols found to be different from received symbols provides a useful indicator of information bit error performance (BER). It is therefore possible to compare this symbol error count (SEC) with a threshold--if the SEC exceeds the threshold, the BFI flag is asserted.
Use of the bit correction threshold with the CRC codeword improves some aspects of the performance of the BFI (i.e., to reduce the probability of missing bad frames), but may also increase the BFI false alarm rate under some circumstances. For example, as the error checking threshold is changed, there is an inverse relationship between the BFI falsing rate performance and carrier to interference (C/I) performance, which directly impacts the sensitivity of the receiver. In other words, the current error checking thresholds are unable to provide sufficient BFI detection and adequate receiver sensitivity simultaneously.
Thus, a need exists for a method and apparatus which provides sufficient BFI detection while maintaining adequate receiver sensitivity in a digital cellular radiotelephone system.